Bolt on exhaust selection help for my 305.

[east302]

A visual might help, for 98 the manual calls one of the upstream sensors on the 305 inactive. Not sure what’s going on there.

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[luconi]

A visual might help, for 98 the manual calls one of the upstream sensors on the 305 inactive. Not sure what’s going on there.

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Well now I'm officially confused.

Just about all of the direct replacement cats for the 5.7l I've been looking all have 3 bungs. Not 4 as shown in your FSM:

https://www.rockauto.com/en/moreinfo.php?pk=10869472&cc=1304873&jsn=480

For the 305 cat... In theory... If bank 1 sensor 1 is inactive... If I were to unplug it would it throw a CEL? Why is it there?

 

[alpinecrick]

Well now I'm officially confused.

Just about all of the direct replacement cats for the 5.7l I've been looking all have 3 bungs. Not 4 as shown in your FSM:

https://www.rockauto.com/en/moreinfo.php?pk=10869472&cc=1304873&jsn=480

For the 305 cat... In theory... If bank 1 sensor 1 is inactive... If I were to unplug it would it throw a CEL? Why is it there?

As I mentioned previously, the 4th bung for the 5.7 is located in the mid pipe section--not the down pipe that holds the two cats that you linked. See Walker part #54090 for the mid pipe.

If you want a factory 5.7 system, your going to have to buy both the down pipe/cat section with 3 bungs and the mid pipe section with 1 bung--for a total of the 4 required O2 bungs.

Just be happy you're not shooting for a 7.4 factory exhaust--it consists on one cat in the down pipe section and one cat in the mid pipe section--and they are both equally expensive......

 

[luconi]

As I mentioned previously, the 4th bung for the 5.7 is located in the mid pipe section--not the down pipe that holds the two cats that you linked. See Walker part #54090 for the mid pipe.

If you want a factory 5.7 system, your going to have to buy both the down pipe/cat section with 3 bungs and the mid pipe section with 1 bung--for a total of the 4 required O2 bungs.

Just be happy you're not shooting for a 7.4 factory exhaust--it consists on one cat in the down pipe section and one cat in the mid pipe section--and they are both equally expensive......

Ah ha. I see now. thank you for pointing that out. I have to buy an entire new exhaust anyway so getting a new mid pipe section is in the cards.

So in theory, I could plug all O2 sensors with what's exiting... The locations would match for at least the pre-cat bungs. However, Would I confuse the computer if I ran the "inactive" sensor that's pre-cat on the 305 to post cat on the 350? I guess I am confused on that portion of it since It's wired and powered to something...

Like I said, just dont want to go throwing CEL's.

 

[df2x4]

However, Would I confuse the computer if I ran the "inactive" sensor that's pre-cat on the 305 to post cat on the 350? I guess I am confused on that portion of it since It's wired and powered to something...

My guess would be yes, that would probably be bad. Pre-cat O2s are used by the PCM to control fuel trims, I wouldn't be comfortable moving one of them post-cat even if it was marked inactive in the manual. Someone at GM put it there for a reason. I'd have to go check in my suburban but I'm pretty sure they both report readings in Torque and DashCommand.

I'm not sure where you're finding that 5.7L mid pipes are cheaper than the 4.3/5.0L ones... The cheapest 4.3/5.0 mid pipe with cats I can find on Summit is a Flowmaster 2010022, it's $292. The cheapest 5.7L one for comparison is a Flowmaster 2010023, it's $337. The 4.3/5.0 cat back systems are a little more expensive than the 5.7L ones for some reason though. Probably because the 5.7L setups are more common and therefore more people sell cheaper versions of them.

Either way Summit has Gibson cat back systems on sale right now. That Flowmaster 2010022 for 4.3/5.0s and a Gibson Supertruck 65514 4.3/5.0 cat back come out to $669. That's a tiny bit over your projected budget but it's all nice stainless steel parts that are correct for your truck and you don't have to worry about O2 sensor placement or anything else weird.

EDIT - That Gibson system is for a short bed though, you never mentioned exactly what truck this is for. They have a long bed system but it's about $80 more.

DOUBLE EDIT - Also realized the Supertruck system is side exit in front of the rear wheel. Probably not what you're looking for. They have different full length systems for a little more $ though.

 

[luconi]

My guess would be yes, that would probably be bad. Pre-cat O2s are used by the PCM to control fuel trims, I wouldn't be comfortable moving one of them post-cat even if it was marked inactive in the manual. Someone at GM put it there for a reason. I'd have to go check in my suburban but I'm pretty sure they both report readings in Torque and DashCommand.

I'm not sure where you're finding that 5.7L mid pipes are cheaper than the 4.3/5.0L ones... The cheapest 4.3/5.0 mid pipe with cats I can find on Summit is a Flowmaster 2010022, it's $292. The cheapest 5.7L one for comparison is a Flowmaster 2010023, it's $337. The 4.3/5.0 cat back systems are a little more expensive than the 5.7L ones for some reason though. Probably because the 5.7L setups are more common and therefore more people sell cheaper versions of them.

Either way Summit has Gibson cat back systems on sale right now. That Flowmaster 2010022 for 4.3/5.0s and a Gibson Supertruck 65514 4.3/5.0 cat back come out to $669. That's a tiny bit over your projected budget but it's all nice stainless steel parts that are correct for your truck and you don't have to worry about O2 sensor placement or anything else weird.

EDIT - That Gibson system is for a short bed though, you never mentioned exactly what truck this is for. They have a long bed system but it's about $80 more.

DOUBLE EDIT - Also realized the Supertruck system is side exit in front of the rear wheel. Probably not what you're looking for. They have different full length systems for a little more $ though.

Ok good to know. I think you talked me out of doing it and sticking with 5.0L designed exhaust equipment. It would have been nice to do some form of an "upgrade" but it seems to not be worth the hassle.

There's a BOSAL 350 downpipe on Rockauto for $267+10 Shipping:

https://www.rockauto.com/en/moreinfo.php?pk=1472038&cc=1304873&jsn=2987

...and yes, aftermarket catbacks are way cheaper for the 5.7 than the 5.0. The Gibson exhaust is sweet but I just don't want to spend that much for a cat back. I think ill just get something OE'ish.

Ill probably get the 5.0L flowmaster CAT and probably just a walker muffler/catback pipe... Assuming the O2 sensors are pretty stuck on the existing pipe, i'm sure ill be getting new ones too.

I have a K1500 extended cab with 6' box.

 

[carguy77]

I've got a magnaflow universal cat on my 96 c1500 with the 4.3. I just bought the same cat for my 96 k1500 with a 305.

C1500 has a custom cat back with a magnaflow chambered muffler.

K1500 has gutted cat(for now) and a magnaflow rumble muffler with a turndown, I scored a borla catback at the JY yesterday for 20 bucks.